In conventional designs for the Hall-Heroult cell, the molten aluminium pool or pad formed during electrolysis itself acts as part of the cathode system. The life span of the carbon lining or cathode material may average three to eight years, but may be shorter under adverse conditions. The deterioration of the carbon lining material is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation by metallic sodium, which causes swelling and deformation of the carbon blocks and ramming mix. Penetration of cryolite through the carbon body has caused heaving of the cathode blocks. Aluminium penetration to the iron cathode bars results in excessive iron content in the aluminium metal, or in more serious cases, a tap-out.
Another serious drawback of the carbon cathode is its non-wetting by aluminum, necessitating the maintenance of a substantial height of pool or pad of metal in order to ensure an effective molten aluminum contact over the cathode surface. In conventional cell designs, a deep metal pad promotes the accumulation of undissolved material (sludge or muck) which forms insulating regions on the carbon cathode surface. Another problem of maintaining such an aluminium pool is that electromagnetic forces create movements and standing waves in the molten aluminium. To avoid shorting between the metal and the anode, the anode-to-cathode distance (ACD) must be kept at a safe 4 to 6 cm in most designs. For any given cell installation, where is a minimum ACD below which there is a serious loss of current efficiency, due to shorting of the metal (aluminium) pad to the anode, resulting from instability of the metal pad, combined with increased back reaction under highly stirred conditions. The electrical resistance of the inter-electrode distance traversed by the current through the electrolyte causes a voltage drop in the range of 1.4. to 2.7 volts, which represents from 30 to 60 percent of the voltage drop in a cell, and is the largest single voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research using Refractory Hard Materials (RHM). such as titanium diboride (TiB.sub.2), as cathode materials has been carried out since the 1950's. Because titanium diboride and similar Refractory Hard Materials which are wetted by aluminium, resist the corrosive environment of a reduction cell, and are excellent electrical conductors, numerous cell designs utilizing Refractory Hard Materials have been proposed in an attempt to save energy, in part by reducing anode-to-cathode distance.
The use of titanium diboride current-conducting elements in electrolytic cells for the production or refining of aluminum is described in the following exemplary U.S. patents: U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061. Despite the rather extensive effort expended in the past, as indicated by those and other patents, and the potential advantages of the use of titanium diboride as a current-conducting element, such compositions have not been commercially adopted on any significant scale by the aluminium industry.
Lack of acceptance of TiB.sub.2 or RHM current-conducting elements of the prior art is related to their lack of stability in service in electrolytic reduction cells. It has been reported that such current-conducting elements fail after relatively short periods in service. Such failure has been associated with the penetration of the self-bonded RHM structure by the electrolyte, and/or aluminium, thereby causing critical weakening with consequent cracking and failure. It is well known that liquid phases penetrating the grain boundaries of solids can have undesirable effects. For example, RHM tiles wherein oxygen impurities tend to segregate along grain boundaries are susceptible to rapid attack by aluminium metal and/or cryolite bath. Prior art techniques to combat TiB.sub.2 tile disintegration in aluminium cells have been to use highly refined TiB.sub.2 powder to make the tile, where commercially pure TiB.sub.2 powder contains about 3000 ppm oxygen.
Moreover, fabrication further increases the cost of such tiles substantially. However, no cell utilizing TiB.sub.2 tiles is known to have operated successfully for extended periods without loss of adhesion of the tiles to the cathode, or disintegration of the tiles. Other reasons proposed for failure of RHM tiles and coatings have been the solubility of the composition in molten aluminium or molten flux, or the lack of mechanical strength and resistance to thermal shock. Additionally, different types of TiB.sub.2 coating materials, applied to carbon substrates, have failed due to differential thermal expansion between the titanium diboride materials and the carbon cathode block or chemical attack of the binder materials. To our knowledge no prior RHM containing materials have been successfully operated as a commercially employed cathode substrate because of thermal expansion mismatch, bonding problems, chemical crosion, etc.
Titanium diboride tiles of high purity and density have been tested, but they generally exhibit poor thermal shock resistance and are difficult to bond to carbon substrates employed in conventional cells. Mechanisms of debonding are believed to involve high stresses generated by the thermal expansion mismatch between the titanium diboride and carbon, as well as aluminium penetrating along the interface between the tiles and the adhesive holding the tiles in place, due to wetting of the bottom surface of the tile by aluminium. In addition to debonding, disintegration of even high purity tiles may occur due to aluminium penetration of grain boundaries. These problems, coupled with the high cost of the titanium diboride tiles, have discouraged extensive commercial use of titanium diboride elements in conventional electrolytic aluminium smelting cells, and limited their use in new cell design. To overcome the deficiencies of past attempts to utilize Refractory Hard Materials as a surface element for carbon cathode blocks, coating materials comprising Refractory Hard Materials is a carbonaceous matrix have been suggested.
In U.S. Pat. Nos. 4,526,911, 4,466,996 and 4,544,469 by Boxall ct al, formulations, application methods, and cells employing TiB.sub.2 /carbon cathode coating materials were disclosed. This technology relates to spreading a mixture of Refractory Hard Material and carbon solids with thermosetting carbonaceous resin on the surface of a cathode block, followed by cure and bake cycles. Improved cell operations and energy savings result from the use of this cathode coating process in conventionally designed commercial aluminium reduction cells. Plant test data indicate that the energy savings attained and the coating life are sufficient to make this technology a commercially advantageous process.
Advantages of such composite coating formulations over hot pressed RHM tiles include much lower cost, less sensitivity to thermal shock, thermal expansion compatibility with the cathode block substrate, and less brittleness. In addition, oxide impurities are not a problem and a good bond to the carbon cathode block may be formed which is unaffected by temperature fluctuations and cell shutdown and restart. Pilot plant and operating cell short term data indicate that a coating life of from four to six years or more may be anticipated, depending upon coating thickness.
The baking process should be carried out in an inert atmosphere, coke bed or similar protective environment to prevent "excessive air burn". In laboratory studies, it is possible to bake the test samples in a retort which maintains a high grade inert atmosphere and excludes air/oxygen ingress; however, this is not practical for commercial use. Baking under a coke bed is reported to give satisfactory protection for the TiB.sub.2 /carbon composite material.
Composite coatings have been tested in plants using full scale aluminium reduction cells (U.S. Pat. No. 4,624,766; Light Metals 1984. pp 573-588; A. V. Cooke et al., "Methods of Producing TiB.sub.2 /Carbon Composites for Aluminium Cell Cathodes", Proceedings 17th Biennial Conference on Carbon, Lexington, Kentucky (1985)). After curing, the coating is quite hard and the coated blocks may be stored indefinitely until baking. For baking, the coated blocks were placed in steel containers, covered with a protective coke bed, and baked using existing plant equipment such as homogenizing furnaces. Once baked, the blocks could be handled without further procautions during cell reline procedures. The integrity of the cured coating and substrate bond remained excellent after baking. No changes in cell start-up procedure were required for using the blocks coated with composite TiB.sub.2 material. No difficulties were encountered when the coated cathode cells were started-up using either a conventional coke resistor bake or hot metal start-up procedure. Core samples from the test cells demonstrated areas of good coating condition after 109 and 310 days of service in the operating cell, but performance was non-uniform.
Extensive testing of TiB.sub.2 /carbon composite materials have been performed in both laboratory and plant tests. The improved laboratory tests and more detailed cell autopsies have shown a variability in material performance not observed in previously reported tests. The x-Ray Diffraction (XRD) analysis was used to measure the trace impurities in the test samples. It was discovered that the poor performance of a test material had a direct correlation with the presence of oxidation products of Ti and B such as TiO and/or TiBO.sub.3, within the structure of the material. A similar variation was detected in the RHM coating applied to a carbon cathode.
Laboratory tests demonstrated that none of the conventional methods (e.g. coke bed, inert gas, liquid metal, boron oxide coating on anodes) for preventing/controlling carbon oxidation was adequate to prevent the formation of TiBO.sub.3 or similar oxidation products during the bake operation and/or the cell start-up.
In addition to the above described problems associated with RHM cathodes, the start-up phase of operation of conventional cells can also result in oxidation damage leading to reduced operational life, and the present invention is not therefore limited to cells have RHM cathodes.